1. Field of the Invention
The present invention relates to image processing methods and image processing devices, and particularly relates to an image processing method and an image processing device which print a plurality of dots to generate a dot set that represents a half-tone level.
2. Description of the Related Art
Printers and printing presses reproduce images by applying ink or toner onto print sheets. A portion where ink or toner is applied represents a darker level in contrast with a portion with no ink or no toner applied thereto representing the lightest level.
When a half-tone level needs to be reproduced, however, it is not easy to control the amount of ink or toner applied onto a print sheet so as to achieve a right amount for a given half-tone level. Because of this, a half-tone is generally represented by controlling a size of an area where ink or toner is applied on a print sheet. This is generally referred to as a pseudo-half-tone scheme.
The pseudo-half-tone scheme includes a screen-dot method, a dither method, and an error-diffusion method, for example.
The screen-dot method is typically used in printing presses. A dot screen film having a periodical variation is superimposed on a transparent film representing an image, and the superimposed image is projected to expose a film. As a result of the exposure, dots are generated in such a fashion as to have area sizes corresponding to transparentness of the original image. An electronic means is utilized these days to generate a dot screen, and screen-dot processing is performed on such an electronic dot screen to generate dots by using a high-resolution image setter.
FIG. 1 is an illustrative drawing showing a circular dot set of the related art.
In FIG. 1, color-dot sets D11 through D15 and white-dot sets D21 through D24 are formed to have circular shapes around predetermined dot-set centers C1 through C9.
In accordance with a specified gray level, an appropriate number of color dots are printed around dot-set centers to produce the color-dot sets D11 through D15, leaving blank areas serving as the white-dot sets D21 through D24. Each of the color-dot sets D11 through D15 is formed to be a circular dot set when the specified gray level corresponds to an area ratio substantially lower than 50%.
As the specified gray level increases, an area ratio of color-dot sets to white-dot sets (blank areas) increases. When the area ratio exceeds 50%, color dots are printed such as to leave circular blank areas, which form the white-dot sets D21 through D24. Namely, the white-dot sets D21 through D24 are formed to be a circular dot set when the specified gray level is substantially higher than the area ratio of 50%.
When the area ratio is smaller than 50%, only those dots which are closer to a center of a color-dot set than to a center of a white-dot set are printed as color dots. In FIG. 1, for example, a dot d0 is closer to the center of the white-dot set D22 than to the center of the color-dot set D13, so that the dot d0 will never be a color dot as long as the area ratio is smaller than 50%. As a result, the color-dot set D13 ends up having a square shape when the area ratio is exactly 50%. The same applies in the case of the other color-dot sets D11, D12, D14, and D15 in FIG. 1.
FIG. 2 is an illustrative drawing showing an ellipse dot set of the related art.
In FIG. 2, the color-dot sets D31 through D35 and the white-dot sets D41 through D44 are formed to have ellipse shapes around predetermined dot-set centers C11 through C19.
In accordance with a specified gray level, an appropriate number of color dots are printed around dot-set centers to produce the color-dot sets D31 through D35, each of which is an ellipse shape when the specified gray level corresponds to an area ratio substantially lower than 50%.
When the area ratio is smaller than 50%, only those dots which are closer to a center of a color-dot set than to a center of a white-dot set are printed as color dots. As a result, the color-dot set D33 ends up having a hexagon shape as shown in FIG. 2 when the area ratio is exactly 50%. The same applies in the case of the other color-dot sets D31, D32, D34, and D35 in FIG. 1.
In the following, the dither method will be described.
The dither method is typically used in printers and display devices having low resolution. While there are several variations to the dither method, an organized dither method is most generally employed.
In this method, a threshold matrix called a dither matrix is used to convert an input image into a dot image by checking whether input-image data exceeds predetermined thresholds.
Choice of a type of a dither matrix determines which one of a half-tone dither image, a dot-concentrated dither image, etc is obtained.
FIGS. 3A through 3G are illustrative drawings showing different types of dither matrixes.
FIG. 3A shows a half-tone-type dither matrix, and FIG. 3B illustrates a screw-type dither matrix.
FIG. 3C exhibits a variation of the screw-type dither matrix, and FIG. 3D demonstrates a half-tone-emphasized-type dither matrix. Further, FIG. 3E shows a dot-concentrated-type dither matrix, and FIGS. 3F illustrates a spiral-dot-type dither matrix. Finally, FIG. 3G shows a square-dot-type dither matrix.
The half-tone-type dither matrix has thresholds assigned to respective dots such that lower thresholds correspond to smaller ordinal numbers shown in FIG. 3A. As a gray level of an image pixel increases, dots are generated in a scattered manner in an ascending order of the ordinal numbers. In this manner, a dot set corresponding to an image pixel is created.
In the screw-type dither matrix and the spiral-type dither matrix, thresholds are assigned such that lower thresholds correspond to smaller ordinal numbers as shown in FIG. 3B and FIG. 3F, respectively, so as to generate dots in a spiral fashion. As a gray level of a pixel increases, dots are generated in an ascending order of the ordinal numbers so as to spread from a center to peripheral areas. A dot set corresponding to an image pixel is created in this manner.
As shown in FIG. 3C, the variation of the screw-type dither matrix has a different order of dot generation at the peripheral portion of the matrix as compared with the original screw-type dither matrix shown in FIG. 3B. In comparison with the original screw type, this variation has a threshold arrangement insuring that a contour shape of the dot set is closer to a circle. Namely, the contour shape of the dot set more closely approximates a circular shape.
In the half-tone-emphasized-type dither matrix, the dot-concentrated-type dither matrix, and the square-dot-type dither matrix as shown in FIGS. 3D, 3E, and 3G, respectively, dot arrangement is concentrated at a center, and is scattered in the peripheral portion of the matrix.
Use of such dither matrixes described above or use of circular/ellipse dot sets previously described generates a plurality of dots that are so concentrated as to form a contiguous dot set. Hereinafter, these methods are referred to as a dot-concentrated-type method.
In the dot-concentrated-type methods, a size of a given dot set gradually increases as an input-image level increases. This results in a better gradation of half-tone representation despite lower resolution of obtained images compared to when other pseudo-half-tone schemes are used. Especially, when an image is printed on a print sheet by using ink or toner, the gradation of half-tone representation is smooth in relation to changes in dot-set sizes, thereby providing a stable representation of half-tone levels.
The dot-concentrated-type methods, however, are rather sensitive to an order of dot generation.
The screw-variation type, the half-tone-emphasized type, the dot-concentrated type, the square-dot type, and the spiral-dot type all suffer a drawback in that a contour shape of a dot set deviates from a circular shape. This results in unstable reproduction of dot shapes on print sheets.
The half-tone type has a disadvantage in that gradation of half-tone representation lacks smoothness because of a scattered dot arrangement.
The spiral-dot type and the screw type have a center of the dot set deviating greatly from an expected center position of the dot set. This causes a problem in that dot sets are not generated at desired positions.
The half-tone-type and the screw type are not based on an established principle about an order in which dots are generated as a gray level increases. This poses a problem when a matrix having a large number of dots needs to be used.
With reference to the circular dot sets of FIG. 1, the number of dots is increased as a gray level increases.
FIGS. 4A through 4C are illustrative drawings for explaining how to generate a circular dot set. FIG. 4A shows dots to be colored in relation to various radii. FIG. 4B shows how the number of dots increases in response to an increase in the radius. FIG. 4C shows a relation between the radius and the number of dots.
As shown in FIG. 4A, four dots d1 through d4 are included (i.e., have centers thereof included) in a circle having a radius r11 around a center which corresponds to one of the dot-set centers C1 through C9 of FIG. 1. Further, eight dots d11 through d18 are included in addition to the four dots d1 through d4 when a radius r12 is considered, bringing the total number of included dots to twelve.
Further, the four dots d1 through d4, the eight dots d11 through d18, and additional four dots d21 through d24, a total number of which is as many as sixteen, are included in a circle having a radius r13. When a circle having a radius r14 is considered, a total of twenty four dots are included, including the four dots d1 through d4, the eight dots d11 through d18, the four dots d21 through d24, and further eight dots d31 through d38. FIG. 4B provides a clear illustration of such a successive increase of printed dots in relation to an increase in the radius.
In this manner, the number of dots increases by a large number at each step of radius increase. This is clearly shown in FIG. 4C. Because of such a large increase at each step, a half-tone representation of a resulting image tends to have conspicuous step-wise changes.
Further, the shape of a dot set may be far from a circle, the degree of which depends on a specified radius. Such a deviation from the circular shape tend to give unstable visual impression.
Accordingly, there is a need for an image processing method and an image processing device which can provide a stable and smooth gradation in half-tone representation even when a large dot set is employed.
Accordingly, it is a general object of the present invention to provide an image processing method and an image processing device which can satisfy the need described above.
It is another and more specific object of the present invention to provide an image processing method and an image processing device which can provide a stable and smooth gradation in half-tone representation even when a large dot set is employed.
In order to achieve the above objects according to the present invention, a method of representing a half-tone level with dot sets each comprised of contiguous dots includes the steps of providing a dot-set center at an offset point that is neither a center of a dot nor a midpoint between centers of dots, selecting dots that are included inside a symmetric-shape area having a center thereof at the dot-set center, an area size of the symmetric-shape area being commensurate with an input image level, and representing a half-tone level corresponding to the input image level by forming a dot set comprised of the selected dots.
According to the method described above, the dot-set center is positioned at the offset position that is neither a center of a dot nor a midpoint between centers of dots, and dots are selected if they are included inside the symmetric-shape area having a center thereof at the dot-set center, an area size of the symmetric-shape area being commensurate with an input image level. With the dot-set center positioned at such an offset position, no two dots have centers thereof equal distance from the dot-set center. This insures that the number of dots in a given dot set increases one by one as the input image level increases, thereby achieving a smooth gradation of half-tone representation.
According to another aspect of the present invention, the symmetric-shape area is circular, and the offset point is positioned such that the dot set comprised of the selected dots maintains a shape as close to a circle as possible. This achieves a stable half-tone representation.
Further, according to another aspect of the present invention, the offset point is positioned as close to a dot center as possible. This insures that the dot sets appear close to their expected positions.